Federal investigators want to know more about the nine-alarm Back Bay fire that took the lives of two Boston firefighters.

The National Institute for Occupational Safety and Health will look into factors that led to the deaths of Lt. Edward Walsh and firefighter Michael Kennedy during the March 26 blaze.

According to institute spokesperson Christina Spring, the CDC-based agency focuses on root causes of fires, not legal or regulatory issues. The agency has looked into previous recent incidents involving Boston fireighter deaths, including the 2007 Tai Ho fire in West Roxbury and a 2009 ladder truck crash in Mission Hill.